1. Field of the Invention
The present invention relates to a method for producing an electrically conductive carbon film.
2. Description of the Related Art
Hitherto, an electrically conductive carbon film has been generally produced by pyrolysis (cf. Otani et al, "Basis for Carbonization Technology" 1980 (Ohm Publishing, Japan)).
This method comprises heating a reaction system in an atmosphere containing a gaseous hydrocarbon such as methane, ethane, propane and benzene to decompose the hydrocarbon and forming a carbon film on a substrate.
A typical carbon film produced by pyrolysis has an electrical conductivity at room temperature of about 70 S/cm, 2 S/cm and 10.sup.-3 S/cm when the pyrolysis temperature is 1,000.degree. C., 700.degree. C. and 500.degree. C., respectively. These results indicate that the electrical conductivity of the carbon film decreases as the pyrolysis temperature decreases, and the carbon film becomes an electrical insulator when the pyrolysis temperature is lower than 500.degree. C. Therefore, by the conventional method, no carbon film having good electrical conductivity has produced at a lower pyrolysis temperature, namely a lower substrate temperature.
To produce a carbon film having a good electrical conductivity at a lower pyrolysis temperature, it has been proposed to use a raw material which decomposes at a lower temperature than the usual hydrocarbons. Otani et al produced a carbon film having an electrical conductivity of 230 S/cm and 180 S/cm on an artificial graphite substrate by pyrolysis of cis-1,2-dichloroethylene at a temperature of 1,100.degree. C. and 700.degree. C., respectively (cf. The Journal of the Chemical Society of Japan, (4) 494 (1979)). M. L. Kaplan et al produced a carbon film having an electrical conductivity of 250 S/cm on a quartz substrate by pyrolysis of 3,4,9,10-perylenetetracarboxylic dianhydride at a temperature of 700.degree. to 900.degree. C. (cf. Appl. Phys. Lett., 36 (10), 867 (1980)). Z. Iqubal et al reported that a carbon film having an electrical conductivity of 15 S/cm and 10.sup.-2 S/cm was formed by pyrolysis of 3,4,9,10-perylenetetracarboxylic dianhydride in a stream of a mixture of argon and hydrogen at a temperature of 800.degree. to 900.degree. C. and 530.degree. C., respectively (cf. Mol. Cryst. Liq. Cryst., 118, 103 (1985)).
On the other hand, to produce a carbon film having a large electrical conductivity on a low temperature substrate, a method other than pyrolysis has been proposed. Mizoguchi et al produced a carbon film having an electrical conductivity of 150 S/cm and 95 S/cm by ion beam deposition on a quartz substrate at a temperature of 1,000.degree. C. and 400.degree. C., respectively (cf. Ext. Abs. Progr. Bienn. Conf. Carbon, 18th., 358-359 and copending U.S. patent application Ser. No. 183,526.).
M. L. Kaplan et al reported that a carbon film having an electrical conductivity of about 10.sup.3 S/cm was produced by ion implantation of 10.sup.17 /cm.sup.2 of Ar ion at 2 MeV into a film formed by vacuum deposition of 3,4,9,10-perylenetetracarboxylic dianhydride (cf. Appl. Phys. Lett., 41(8), 708 (1982)).
According to the conventional pyrolysis, when the hydrocarbon such as methane and ethane is used as a raw material, the carbon film having the electrical conductivity of 70 S/cm was produced at 1,000.degree. C., while the carbon film having the electrical conductivity of only about 10.sup.-3 S/cm was produced at 500.degree. C. Therefore, at a lower substrate temperature, the electrical conductivity of the carbon film greatly decreases.
When 3,4,9,10-perylenetetracarboxylic dianhydride which is more easily decomposed than the hydrocarbons is used, the electrical conductivity of the carbon film is slightly increased to 15 to 250 S/cm at a temperature of 700.degree. to 900.degree. C. and to 10.sup.-2 S/cm at 530.degree. C. But, this increase of the electrical conductivity is not significant.
When cis-1,2-dichloroethylene is used as a raw material, the carbon film has the electrical conductivity of 230 S/cm at 1,100.degree. C. and 180 S/cm at 700.degree. C. However, a corrosive gas is generated during pyrolysis, which is not practically acceptable.
In case where the ion beam is used for producing the electrically conductive carbon film, the carbon film has a comparatively large electrical conductivity at a synthesis temperature of 400.degree. C. or higher. However, the hydrocarbon such as benzene is used as a raw material, no electrical conductive carbon film is produced at a synthesis temperature of lower than 400.degree. C.
Further, the carbon film which is produced at room temperature by ion implantation in the vacuum deposited film of 3,4,8,10-perylenetetracarboxylic dianhydride has a large electrical conductivity of 10.sup.3 S/cm. But, this technique requires two production steps, namely vacuum deposition and ion implantation and large equipment.